Conversion of Large-Bandgap Triphenylamine-Benzothiadiazole to Low-Bandgap, Wide-Band Capturing Donor-Acceptor Systems by Tetracyanobutadiene and/or Dicyanoquinodimethane Insertion for Ultrafast Charge Separation

Abstract

Usage of multimodular donor-acceptor systems capable of revealing tunable ground- and excited-state properties is gaining momentous interest for applications in light energy harvesting and optoelectronics. Here, we demonstrate conversion of a large-bandgap donor-acceptor-donor (D-A-D) type system, (triphenylamine-benzothiadizole-triphenylamine, TPA-BTD-TPA) into low-bandgap, unsymmetrical, D-A′-A-D and D-A′-A-A″-D type donor-acceptor systems by the insertion of tetracyanobutadiene (A′) or dicyanoquinodimethane (A″) by [2 + 2] cycloaddition-retro-electrocyclization reactions. Because of the existence of strong charge transfer in the ground and excited states, these low-bandgap unsymmetrical donor-acceptor chromophores exhibit strong electronic absorption covering the visible and near-IR regions. Electrochemical, spectroelectrochemical, and computational studies are performed to evaluate their redox potentials and spectral characterization of oxidized/reduced species as well as to realize their electronic structures. Finally, the occurrence of ultrafast charge separation in these conjugates has been established from femtosecond transient absorption covering the visible-near-IR regions in polar and nonpolar solvents - properties relevant toward their optoelectronic applications. Copyright © 2019 American Chemical Society.